Trench structures, such as shallow trench isolations (STIs), are used to separate and isolate active areas on a semiconductor wafer from each other. As circuit densities continue to increase, the widths of trenches of STIs decrease, thereby increasing the aspect ratios of the STI trenches. Aspect ratio of a trench (or a gap) is defined as the trench height (or gap height) divided by the trench width (or gap width). Incomplete gap-filling results in unwanted voids and increases the risk of inclusion of unwanted defects when the unwanted voids are exposed during removal of excess dielectric. The voids may also result in inadequate isolation between active areas. The presence of voids in STI would affect yield.